Memory devices, such as conductive bridging random access memories (CBRAMs), can have memory elements that can be programmed between high and low resistance values to store data. By application of an electric field, CBRAM type memory elements can form conductive regions (i.e., filaments) through a solid electrolyte layer to place the memory element in a low resistance state. A conductive filament can include clusters of metallic elements. Application of an opposing electric field can dissolve such filaments to place the memory element into a high resistance state.
Performance features of CBRAMs, and similar devices, can include data retention and endurance. Data retention is be the amount of time a memory element can retain a data value after being programmed (i.e., placed into a particular conductive state). Endurance is the number of times a memory element can be programmed to one or more states before its data retention is adversely affected. It is understood that the term “programmed” denotes those operations that place a memory element into a high resistance state as well as a low resistance state.
FIGS. 14A to 14D are diagrams representing potential degradation mechanisms of metal atom clusters that can make up conductive filaments in a solid electrolyte based memory element. FIGS. 14A to 14D show clusters of metallic elements (in this example silver) in a solid electrolyte (e.g., an ion conducting material).
FIGS. 14A and 14B show a metallic cluster 1401 of three silver atoms that can be formed within a solid electrolyte by application of an electric field. Clusters 1401 (and others like it) can establish a data state by creating conductive paths within the solid electrolyte. FIGS. 14A and 14B also show how such clusters 1401 can oxidize into smaller clusters 1405.
Referring to FIG. 14A, section 1470-0 shows an example of a metal atom 1403 that is oxidized, and then migrates from away from the cluster 1401 in a solid electrolyte layer. Section 1470-1 shows how a remaining electron can also migrate leaving the smaller cluster 1402, as shown in section 1470-2.
Referring to FIG. 14B, section 1470-3 show an example of a metal atom 1403 that is oxidized and initially remains with the cluster 1401 in an ionic form. Section 1470-4 shows how a metal ion can then subsequently migrate away from the cluster to form a smaller cluster 1405, as shown in section 1470-5.
FIGS. 14C and 14D show how a metallic cluster 1405 of two silver atoms can be oxidized to a point where the cluster is essentially dissolved (e.g., removed from a filament region). FIGS. 14C and 14D show a “trap” 1407 that can exist in a solid electrolyte material. A trap 1407 can present a structure within the solid electrolyte material conducive to the formation of a cluster.
FIG. 14C shows an example of the oxidation of a first atom, with migration of the ion out of the cluster (section 1473-0), followed by movement of the free electron out of the trap 1407 (section 1473-1). A remaining atom can oxidize (section 1473-2) and migrate, followed by the electron (section 1473-3). Consequently, the cluster can disappear, as shown in section 1473-4.
FIG. 14D shows another example of the oxidation of a first atom, with migration of the electron (section 1473-5) followed by the ion (section 1473-6). A remaining atom can oxidize (section 1473-7) and migrate (1473-8), resulting in dissolution of the cluster, as shown in section 1473-4.
It is understood that the oxidation described above can be intentionally induced by application of an electric field opposite to that which established the cluster. However, it is also believed that such clusters can degrade when not intended, which can adversely affect data retention and/or endurance of a memory element.
FIGS. 15A to 15C show data writing operations for a conventional CBRAM type memory element. A memory element 1509 has a solid electrolyte layer 1511 formed between an active electrode 1513, and an indifferent electrode 1515. It is understood that solid electrolyte layer 1511 includes an oxidizable metal incorporated within. Further, an active electrode 1513 can be a further source of the oxidizable metal.
FIG. 15A shows how the application of an electric field (+Vprog to −Vprog), that is positive in a direction from the active electrode to the indifferent electrode, can result in the oxidation of the metal (M) within solid electrolyte layer 1511.
FIG. 15B shows how application of an electric field can create a higher conductive structure 1517 or “filament” between the electrodes (1513/1515). Creation of such a filament can alter a conductivity through solid electrolyte layer 1511. A read voltage (+Vread to −Vread) can be applied to sense a conductivity across the electrodes.
Referring still to FIG. 15B, a portion 1519 of the conductive structure 1517 closer to the active electrode 1513 can have smaller metal atom clusters than portions closer to the indifferent electrode 1515. Consequently, portion 1519 can be more susceptible to degrading (e.g., oxidizing and then dissolving away), which can undesirably increase a resistance through solid electrolyte layer 1511.
FIG. 15C shows how the application of an electric field opposite to that of FIG. 15A (−Verase to +Verase) can result in the oxidation of the metal (M) within solid electrolyte layer 1511 that can reduce and/or substantially eliminate the conductive structure 1517.